In recent years, reduction in the amount of carbon dioxide CO2 output has been desired ardently from the viewpoint of global environmental conservation. In particular, in the automobile field, reduction in the weight of a car body has been required strongly to enhance fuel economy and reduce the amount of CO2 output. Such circumstances are the same in the use of a steel sheet, and demands for reduction in the usage of a steel sheet which exhibits a large amount of CO2 output in production of the steel sheet have increased.
In particular, as for a structural member, where deformation of a part should be avoided, reduction in thickness through enhancement of the yield strength of a steel sheet is effective from the viewpoint of reduction in the usage (mass) of the steel sheet. However, if the yield strength of the steel sheet is enhanced, there is a problem that shape defects occur because of springback or the like in press forming. When shape defects occur, it is necessary that a press forming step be further added to correct the shape by press forming into a predetermined shape. The shape correction increases the production cost and, in addition, particularly in high strength steel sheets having a yield strength of 1,000 MPa or more, it may become impossible to correct the shape up to a predetermined shape. Consequently, the impossibility of improving the shape fixability of the high strength steel sheet becomes a hindrance to an achievement of reduction in thickness of the high strength steel sheet.
Then, a ferrite phase which is soft, easy to press form, and advantageous to ensure the shape and a martensite phase which is hard and advantageous to enhance the strength are combined and, thereby, a dual-phase steel sheet has been developed as a high strength steel sheet having good shape fixability and a high tensile strength in combination. However, although that technology can enhance tensile strength, there is a problem that the yield strength is reduced because of the presence of the soft ferrite phase. To enhance the yield strength of the above-described dual-phase steel sheet, it is necessary that the microstructure has a very high percentage of martensite phase. But, as for the dual-phase steel sheet having such a microstructure, a new problem is induced in that cracking occurs during press forming.
For example, Japanese Patent No. 4464748 describes a high strength steel sheet having excellent shape fixability and stretch flangeability as the high strength steel sheet having improved shape fixability. The high strength steel sheet described in Japanese Patent No. 4464748 has a chemical composition containing C: 0.02% to 0.15%, Si: more than 0.5% and 1.6% or less, Mn: 0.01% to 3.0%, Al: 2.0% or less, Ti: 0.054% to 0.4%, and B: 0.0002% to 0.0070% and further containing at least one of Nb: 0.4% or less and Mo: 1.0% or less on a percent by mass basis. Also, the high strength steel sheet described in Japanese Patent No. 4464748 has a microstructure in which the greater part of phase is ferrite or bainite, and a texture in which an average value of X-ray random intensity ratios of {001}<110> to {223}<110> orientation groups of a sheet face at the position one-half the sheet thickness is 6.0 or more, and an X-ray random intensity ratio of at least one of the {112}<110> orientation and the {001}<110> orientation among these orientation groups is 8.0 or more. Also, the high strength steel sheet described in Japanese Patent No. 4464748 has a microstructure in which the number of compound precipitates having a particle size of 15 nm or less is more than or equal to 60% of the total number of the compound precipitates, and at least one of the r value in the rolling direction and the r value in the direction at a right angle to the rolling direction is 0.8 or less. It is mentioned that according to Japanese Patent No. 4464748, a steel sheet having highly improved shape fixability and excellent hole expansion property is obtained by adjusting the precipitates and the texture at the same time.
Meanwhile, Japanese Unexamined Patent Application Publication No. 2008-174805 describes a high yield strength hot rolled steel sheet. The hot rolled steel sheet described in Japanese Unexamined Patent Application Publication No. 2008-174805 has a chemical composition containing C: more than 0.06% and 0.24% or less, Mn: 0.5% to 2.0%, Mo: 0.05% to 0.5%, Ti: 0.03% to 0.2%, V: more than 0.15% and 1.2% or less, and Co: 0.0010% to 0.0050% on a percent by mass basis. Then, the hot rolled steel sheet described in Japanese Unexamined Patent Application Publication No. 2008-174805 has a microstructure which is substantially a ferrite single phase and in which complex carbides containing Ti, Mo, and V and carbides containing V only are dispersed, where the total of the amount of Ti precipitated as complex carbides containing Ti, Mo, and V and the amount of V precipitated as carbides containing V only is more than 0.1000% and less than 0.4000% on a percent by mass basis. Also, the hot rolled steel sheet described in Japanese Unexamined Patent Application Publication No. 2008-174805 has a high yield strength of 1,000 MPa or more. It is mentioned Japanese Unexamined Patent Application Publication No. 2008-174805 that a high yield strength steel sheet having highly improved bending property after working and a yield strength of 1,000 MPa or more is obtained because a very small amount of Co is contained, substantially a ferrite single phase is present, and complex carbides containing Ti, Mo, and V and carbides containing V only are dispersed.
However, according to Japanese Patent No. 4464748, the compound (precipitate) particle size is large and the resulting yield strength is up to about 900 MPa. That is, according to Japanese Patent No. 4464748, it is difficult to further enhance the yield strength up to 1,000 MPa or more. Meanwhile, according to Japanese Unexamined Patent Application Publication No. 2008-174805, the bending property after working is improved, but the problem remains in that a predetermined shape fixability cannot be ensured.
It could therefore be helpful to provide a high strength steel sheet having a yield strength of 1,000 MPa or more and excellent shape fixability, and a method of manufacturing the same. In this regard, the yield strength YP of the high strength steel sheet should preferably be 1,100 MPa or more, and further preferably 1,150 MPa or more. Here, the thickness of the “steel sheet” is 2.0 mm or less, preferably 1.7 mm or less, more preferably 1.5 mm or less, and further preferably 1.3 mm or less.